Russia struck a military target on the edge of Kyiv overnight with cruise missiles and has taken control of a steel plant in Mariupol, the Defense Ministry said Friday, pledging more strikes against the Ukrainian capital Kyiv after losing its Black Sea fleet flagship.

Capital Kyiv was hit on Friday by some of the most powerful explosions heard since Russian forces withdrew from the area two weeks ago. Russia's Defense Ministry said in a statement that its overnight missile strikes on Kyiv had struck the "Vizar" factory on the edge of the Ukrainian capital, which it said made and repaired missiles, including anti-ship missiles. It pledged more strikes on Kyiv.

"The number and scale of missile strikes on targets in Kyiv will increase in response to any terrorist attacks or acts of sabotage on Russian territory committed by the Kyiv nationalist regime," the Russian Defense Ministry said in a statement.

It said its forces had shot down a Ukrainian Mi-8 helicopter, which it said had attacked the village of Klimovo in the Bryansk region on April 14 and had also shot down a Ukrainian Sukhoi-27 jet. A group of up to 30 Polish mercenaries had also been destroyed, it said.

The ministry also said its forces had captured the Ilyich steelworks in the besieged port city of Mariupol, one of the last industrial areas holding out in the besieged eastern city that has seen the war's heaviest fighting and the worst humanitarian catastrophe.

For its part, Ukraine said it had repelled Russian offensives in the town of Popasna and Rubizhne, in an area north of Mariupol. Both reports could not be independently confirmed.

The explosions were reported to have been heard after the Russian Defense Ministry announced that the Moskva, the flagship of Russia's Black Sea Fleet, had sunk while being towed after being badly damaged.

Ukraine claimed the Moskva's damage was the result of one of its missile strikes, while Russia's Defense Ministry spoke only of a fire breaking out and of exploding ammunition. Although Russia did not acknowledge that Ukrainian missiles had hit the ship, early on Friday it struck what it described as a factory in Kyiv that made and repaired anti-ship missiles, in apparent retaliation.

The Moskva was by far Russia's largest vessel in the Black Sea fleet, equipped with guided missiles to attack the shore and shoot down planes, and radar to provide air defense cover for the fleet.

Russia launched what it calls its "special military operation" on Feb. 24. Ukraine has put up fierce resistance and the West has imposed sweeping sanctions on Russia. On the first day of the war, the ship ordered Ukrainian defenders of an island outpost to surrender and they radioed back an obscenity, an event marked on a postage stamp that Kyiv released hours before saying it had struck it.

Zelenskyy told Ukrainians late Thursday that they should be proud of having survived 50 days under Russian attack when the invaders "gave us a maximum of five." Back then even friendly world leaders urged him to leave, unsure whether Ukraine could survive, he said: "But they didn’t know how brave Ukrainians are, how much we value freedom and the possibility to live the way we want."

Listing the ways Ukraine has defended against the onslaught, Zelenskyy addressed "those who showed that Russian warships can sail away, even if it’s to the bottom" of the sea.

It was his only reference to the guided-missile cruiser Moskva, named after the Russian capital, which became a potent target of Ukrainian defiance in the opening days of the war. It sank Thursday while being towed to port after suffering heavy damage under circumstances that remained under dispute.

Russia has used its naval power to blockade Ukrainian ports and threaten a potential amphibious landing along the coast. Without its flagship, its ability to menace Ukraine from the sea could be crippled.

"If reports of Moskva’s sinking prove true it will be emblematic of Russia’s overall military effort thus far," tweeted Michael Kofman, an expert on Russia's military, who called it a "major loss for the Russian navy."

No warship of such size has been sunk during a conflict since Argentina's General Belgrano, which was torpedoed by the British in the 1982 Falklands war.

Battle for Mariupol

Kyiv and its allies say Russia has launched an unprovoked war that has seen more than 4.6 million people flee abroad and killed or wounded thousands.

Russia said on Wednesday that more than 1,000 Ukrainian marines from one of the units still holding out in Mariupol had surrendered. Ukrainian officials did not comment.

If taken, Mariupol would be the first major city to fall to Russian forces since they invaded, allowing Moscow to reinforce a land corridor between separatist-held eastern Donbass areas and the Crimea region it seized and annexed in 2014.

Ukraine said tens of thousands of people were believed to have been killed in Mariupol, where efforts were underway to evacuate civilians.

The Black Sea port, home to 400,000 people before the war, has been reduced to rubble by seven weeks of siege and bombardment, with tens of thousands of people trapped inside.

Russia's Defense Ministry said late on Thursday that 815 people had been evacuated from the city over the past 24 hours. Ukraine said that figure was 289.

Moscow now says its main war aim is capturing the Donbass, an eastern region of two provinces that are already partly held by Russian-backed separatists and that Russia wants Kyiv to cede. It has sent a new column of thousands of troops into the east for what Ukraine anticipates will be a major assault. Moscow says it hopes to seize all of Mariupol soon, which would be the only big city it has captured so far.

Russia initially described its aims in Ukraine as disarming its neighbor and defeating nationalists there. Kyiv and its Western allies say those are bogus justifications for an unprovoked war of aggression that has driven a quarter of Ukraine's 44 million people from their homes.